In a spark plug for use in an internal combustion engine, a ground electrode is welded to a combustion chamber side front end portion of a metal shell which holds an insulating member (an insulator) in which a central electrode is inserted, so that a free end portion of the ground electrode is allowed to confront the front end portion of the center electrode so as to form a spark discharge gap. Then, an electric spark is discharged between the center electrode and the ground electrode, which ignites an air/fuel mixture exposed between both the electrodes to thereby form a flame kernel.
In assembling a metal shell and an insulator into a spark plug, a front end portion of the insulator is inserted into the metal shell from a rear end side towards a front end side of the metal shell, and an opening portion at the rear end side of the metal shell is crimped towards the insulator side (radially inwards of the metal shell). Then, in order to ensure gastightness between the metal shell and the insulator, an annular metallic packing is loaded between the metal shell and the insulator as a seal member (refer to Patent Document 1, for example).
FIG. 8 shows a seal portion of a spark plug described in Patent Document 1. As is shown in FIG. 8, a ledge portion 202 is formed on an inner wall of a metal shell 200 which is inclined by an inside diameter being reduced gradually towards a front end portion thereof, and a step portion 206 is formed on an outer wall of an insulator 204 which is inclined by an outside diameter being reduced gradually towards a front end portion thereof and which confronts the ledge portion 202 of the metal shell 200. In addition, an iron packing 208 is loaded between the ledge portion 202 and the step portion 206 as a seal member.